A vehicle brake control device is disclosed in JP 2000-203401A as a known motion control device. In the device, each pump supplies the brake fluid to generate brake fluid pressure, and the brake fluid pressure is supplied to wheel cylinders (hereinafter, referred to as W/C) for automatic pressurization. In each conduit connecting a master cylinder (hereinafter, referred to as M/C) and the W/Cs, a pressure difference control valve is disposed close to the M/C relative to a connecting point of the conduit connecting between an outlet port of each pump and the W/C. The pressure difference control valve is adjusted to generate a predetermined pressure difference in the conduit or to shut off the conduit. Additionally, the pressure increasing control valve(s), each controlling the pressure increase to the corresponding W/C of each wheel to be controlled (hereinafter, referred to as controlled wheel), is (are) controlled to be opened or closed by duty ratio, and the pressure increasing control valve corresponding to each non controlled wheel is closed. As a result of the movements of these valves, the pump suctions or discharges the brake fluid. Accordingly, the pump supplies the brake fluid to generate the brake fluid pressure, thereby pressurizing the W/C of each controlled wheel.
In this type of brake control device, the amount of the current supplied to the motor is controlled in order to reduce operating noise occurring when the pump discharges. For example, the amount of the current supplied to the motor, which drives the pumps, is controlled by duty ratio so as to be small when a frictional coefficient of a road surface is relatively small, compared to when the frictional coefficient is relatively large, thereby reducing the driving amount of the pump, i.e. the pump suction/discharge amount. Further, as the actual W/C pressure becomes closer to a target pressure, the amount of the current being supplied to the motor is limited to reduce the driving amount of the pump.
However, the pressure difference control valve of each controlled wheel is brought into a predetermined pressure difference generating state or a fluid communication interrupting state, and the pressure increasing control valve, which controls the pressure increase to the W/C of each non-controlled wheel, is closed for pressurizing the W/C of each controlled wheel. Thus, the brake fluid pressure becomes high around the outlet port of the pump. Therefore, when the pump suction/discharge amount is limited and the brake fluid pressure becomes high around the outlet port of the pump, the driving of the pump is restrained.
A need exists for a vehicle motion control apparatus which reduces operating noise in view of a load condition of a motor.